1. Field of the Invention
The present invention relates to an optical disk device and can be applied to, for example, a reproduction system of a phase-change-type optical disk device handling high density record. In the invention, a certain pattern is detected using NRZI data that is NRZI-modulated from pulse-position-modulated data obtained by signal processing of reproduction signals, thereby improving the accuracy of detecting certain patterns, such as a synchronizing pattern, without increasing the redundancy of the format.
2. Description of the Related Art
In recent years, in an optical disk device for high density recording, due to inevitable intersymbol interference in the reproduction data, data recorded in an optical disk is reproduced by PRML (Partial Response Maximum Likelihood) using a maximum likelihood decoding circuit such as a viterbi decoding circuit.
In this type of optical disk device, user data to be recorded is divided into block units of a predetermined size, has error correction symbols and the like added thereto, and then a process including interleaving is performed. Subsequently, each block has a synchronizing pattern and so on allocated thereto. Further, such generated data rows are modulated into, for example, RLL (Run Length Limited) data, and then this RLL data is NRZI (Non Return to Zero Inverted) modulated. In an optical disk device such as a phase-change-type optical disk, the light intensity of a laser beam is increased with a predetermined timing, according to the NRZI data, thereby sequentially forming a string of marks to record desired data onto an optical disk. In an optical magnetic disk device, the light intensity of a laser beam is increased with a constant period, and a modulated magnetic field is applied to the positions irradiated by the laser beam, according to NRZI data, thereby forming a string of marks to heatmagnetically record desired data.
When reproducing data, a laser beam is irradiated to an optical disk and the returning light is received, thereby generating reproducing signals having a signal level varying according to the a string of marks. In an optical disk device, this laser beam forms beam spots with a predetermined size on an optical disk to cause intersymbol interference; accordingly, reproducing signals with the intersymbol interference are detected in the presence of superimposed random noise, that is to say, Gaussian noise.
In the optical disk device, the reproducing signals are binarized in such a manner as to reproduce a clock, and then the reproducing signals are converted from analog to digital, using this clock to generate digital reproducing signals. Further, the digital reproducing signals are decoded by a viterbi decoder to obtain RLL data. This RLL data is pulse-position-modulated data in which pits and lands, or marks and spaces are represented by a logical value. In the optical disk device, with the synchronizing pattern detected by this RLL data as a reference, the RLL data is selectively taken to be RLL-decoded, and then deinterleave processing, error correction processing, and the like are performed to reproduce the user data recorded on the optical disk.
Since synchronizing patterns are used as processing references for selecting from RLL data, improved detection accuracies are desired. For this purpose, it is necessary to assign a pattern that does not occur in other parts as a synchronizing pattern to extend the length of the pattern, and to determine the reproduced RLL data, using the long pattern length.
However, using a long pattern length to improve the detection accuracy of the synchronizing pattern causes a problem in that the redundancy of the format is increased instead, which prevents the data recording surface of the optical disk from being used for recording user data effectively.